The present invention relates in general to radio frequency (RF) signal filters, and more particularly to an improved ceramic bandpass or band-elimination filter that is particularly well adapted for use in radio transmitting and receiving circuitry. Further, the present invention provides dielectric multi-resonator filters including a plurality of resonators that are quarter-wavelength coaxial transimission lines. The present invention is best applicable to a bandpass filter in which adjacent resonators are coupled by a coupling means and a capacitive reactance element is provided in parallel to the coupling means to provide an attenuation pole or poles.
A conventional multi-resonator filter in a unitary structure of a single dielectric block is shown in Japanese Patent Publication (unexamined) No. 63-64404 published Mar. 22, 1988, in which a plurality of resonators are formed in a dielectric block. A coupling device for coupling the adjacent resonators and a reactance element, in parallel to the coupling device, are provided for forming an attenuation pole, so that greater attenuation out of the band is obtained without increasing the number of resonators.
In the conventional dielectric filter disclosed in the above-stated Japanese publication, the filter includes a block which is comprised of a dielectric material that is selectively plated with a conductive material. The dielectric block has four resonator holes, which each extend from the top surface to the bottom surface thereof. The resonator holes are likewise plated with an electrically conductive material to provide an inner conductor. The dielectric block has coupling holes between the adjacent resonator holes. The dielectric block is covered or plated with a conductive material with the exception of one side where the resonator holes are opened to thereby form an outer conductor so that the inner conductor and the outer conductor are short-circuited by a metalized layer on one side of the dielectric block. A resonator is formed by the inner conductor, outer conductor and a dielectric block portion between the inner and outer conductors.
Resin molded terminals are fitted to the resonator holes, the resin molded terminals being formed of metal pins with resin enclosed therearound. A predetermined electrostatic capacitance is formed between the metal pins and respective inner conductor, and the metal pins are connected to each other by a substrate having a conductive pattern. One resin molded terminal constitutes a coupling capacitance and the other resin molded terminal constitutes a capacitive reactance element. By addition of the coupling capacitance, an attenuation pole is formed to thereby increase the amount of attenuation of frequency out of the band.
Basically, a value of a coupling reactance for providing an attenuation pole is generally very small, for example 1 pF or less, and a very small variation or change in the coupling reactance results in changes in filter characteristics. Accordingly, in the conventional filter structure as disclosed in the aforementioned Japanese publication wherein the resin molded terminals are reactance elements, there are numbers of variation factors such as shape or diameter of the resonator holes, fitting dimensions of the resin, etc. and there is an inconvenience in adjusting the coupling.